The preferred embodiment of the present invention operates a low voltage brushless DC spindle motor in a magnetic disc drive storage unit. For that reason, the background of this invention will be described with respect to operation of such motors. However, the present invention may be used to control brushless DC motors in non-computer applications as well.
As shown by FIG. 1, a typical magnetic disc storage system 2 includes one or more magnetic storage discs or platters 4, 6 that are rotated with run velocity .omega. by a spindle motor 8 electrically coupled to drive electronics 10. Discs 4, 6 each have upper and lower surfaces upon which data may be magnetically written or read. More specifically, projecting arms of an actuator carriage 12 carry read/write heads (hereafter "heads") 14, 16 that respectively read and/or write data from the surfaces of discs 4 and 6. Generally, a voice coil mechanism 18 causes actuator carriage 12 to move all heads radially under command of a positioning servo controller mechanism (not shown).
When the spindle motor is not running, disc drive units park the head-ends of the actuator carriage. Some disc drive units include stationary parking ramps (not shown), upon which the head-ends of the actuator carriage 12 rest when the spindle motor 8 is not running. It may in fact be advantageous to couple the so-called back electromagnetic force ("EMF") generated by the spindle motor 8 during power-down (e.g., when motor 8 is turning off) to the voice coil mechanism to assist positioning carriage 12 onto the ramps. Alternatively, many disc drive units provide a latching mechanism (not shown) that engages the head-end of the actuator carriage when the spindle motor is not operating. So-called inertia latches (not shown) may also be provided to engage the head-end of the actuator carriage in the event of mechanical shock. It will be appreciated that parking the heads away from the disc surfaces can help protect the disc media from damage from physical contact with the heads. In any event, at spindle motor power-up, the heads must be unparked.
Modern disc drive units must also be able to handle substantial mechanical shock, often several hundred "g" units, or higher. To enhance mechanical shock handling capacity, the actuator assembly 12 is mechanically preloaded, essentially to stiffen the assembly. In disc drive units without stationary parking ramps, more stiffening is generally required to protect the magnetic media on the disc platter surfaces against physical contact with the heads.
Brushless DC low voltage motors such as spindle motor 8 are well known in the art. Such motors have a number of windings (or "phases"), that are sequentially coupled to a direct current ("DC") power source. As current flows through selected windings, torque-inducing magnetic flux orientations are produced in a synchronized fashion. The resultant torque causes a desired rotational movement of the motor rotor and attached discs 4, 6.
FIG. 2 shows the internal motor windings A, B and C for a typical three-phase brushless DC spindle motor 8. Each winding has an inductance Lx effectively in series with an associated resistance Rx and an effective back EMF generator Ex, where x=a, b or c. Further, effective mutual inductances, shown as m ab, m bc, m ac, are also present between the windings. Collectively, for each winding A, B or C, the associated inductance, series resistance, and mutual inductance may be defined by an effective impedance Zx in series with an associated back EMF generator Ex.
Drive electronics 10 typically provides two output drive transistors per each winding, six transistors Q1-Q6 in total. The drive transistors need not be bipolar (as shown), and could in fact be field effect devices, or any other switching device. Low level drive circuitry (not shown) within electronics 10 generates sequential base drive signals that are coupled to the base leads of drive transistors Q1-Q6. These base drive signals sequentially pulse selected drive transistors to produce the desired motor rotation.
In the prior art, motor 8 is operated by simultaneously energizing two series-coupled phases, a so-called "line-to-line" or "bipolar" mode of operation. In this sense, "bipolar" means that current through a winding may be caused to flow in either direction, as contrasted to a "unipolar" configuration wherein winding current can only flow in one direction. To achieve line-to-line mode, output drive transistors Q1-Q6 are sequentially switched to maintain two motor windings coupled in series between the power supply Vcc and ground. For example, by simultaneously providing positive base signals to Q1 and Q6, these two drive transistors turn on, energizing windings A and B, all other drive transistors being off. After flux produced by energized windings A and B causes the rotor of motor 8 to rotate 60.degree. (electrical degrees), the low level drive circuitry turns off Q1 and turns on Q2, whereupon windings B and C are energized. After 60.degree. further electrical rotation, Q6 is turned off and Q1 turned on to energize windings C and A, and so forth.
Line-to-line operation has the advantage of generating a large torque when the spindle motor is turned on ("power-up"). A large initial torque is required to overcome frictional and other forces to ensure start-up rotation of the spindle motor rotor and attached discs 4 and 6.
The following two equations must be taken into account when operating a spindle motor: EQU T.sub.x =Kt.sub.x .multidot.I.sub.x ( 1) EQU Ix=(Vcc-EMF.sub.x)/Z.sub.x ( 2)
where for each winding number x, T.sub.x =net torque, e.g., torque less detent torque T.sub.D (ounce-inch), Kt.sub.x =spindle motor torque constant (ounce-inch/ampere), I.sub.x =winding current (amperes), Vcc=power supply voltage (volts), EMF.sub.x =back EMF (volts), and Z.sub.x =equivalent winding impedance (ohms).
Detent torque T.sub.D is the torque that exists in the spindle motor when none of the windings are energized. This torque results from magnetically-related motor torque due to mechanical misalignment and the like, as well as from coulomb friction torque, and magnetic loss torque.
In applying equations (1) and (2), it is understood that the torque, Kt, and back EMF contribution of each energized series-coupled winding a vector component to the resultant produced torque, Kt, and back EMF. For example, in a three-phase motor, the vector resultant produced by two-series coupled windings, each having a unit torque constant Kt, will be Kt.multidot..sqroot.3 or 1.73.multidot.Kt, assuming sinusoidal back EMF waveforms.
When the spindle motor is off, back EMF is of course zero. However, at power-up, back EMF increases with increasing rotor rotational velocity .omega.. Thus, at power-up, little back EMF exists to retard current I. As a result, for a given Vcc (e.g., 5 VDC, or 3.3 VDC or even 3.0 VDC), a large start-up current I can be provided. From equation (1), it is apparent that by series-coupling two (or even more) windings that each have a reasonably large Kt, the resultant vectorially-combined Kt will be larger that the Kt for a single winding. As noted, the vector sum produced by series-coupling two windings in a three-phase (e.g., 120.degree. offset) motor is 1.73.multidot.Kt. Thus, the resultant combination Kt.times.I product will be large, and the generated start-up torque T can be sufficiently large to ensure that the spindle motor will indeed rotate at power-up.
As noted, a large Kt is advantageous for spindle motor power-up. But, unfortunately, too large a Kt can become a liability once the desired rotational run velocity .omega. is attained because of the excessively large back EMF produced by the combined windings. As run velocity .omega. is approached and then attained, each series-coupled winding contributes an increasingly substantial back EMF component.
Since the available run current is directly proportional to (Vcc-back EMF), too a large back EMF (resulting from too large a Kt) can substantially reduce the magnitude of the available run current. Stated differently, as back EMF increases with rotational velocity, drive electronics 10 may no longer be able to cause sufficient current flow through the energized windings to sustain spindle motor rotation. Ideally, Kt should be sufficiently large to maintain a reasonable run current.
For a given Kt motor, Vcc may have to be increased to ensure a sufficient magnitude of run current through the series-coupled windings. However, increasing Vcc may not be a viable option as disc drive system power consumption will be increased substantially. Further, it may not be possible to increase Vcc. Many modern hard disc drive systems are expected to operate reliably and efficiently from a fixed low voltage battery-operated power supply, wherein Vcc may be as small as 3.0 VDC. Clearly, ensuring efficient normal run speed operation of a high Kt spindle motor from a low voltage power source may not always be feasible.
A method of operating a spindle motor at a lower voltage Vcc was disclosed in concurrently filed U.S. patent application Ser. No. 08/205,185 filed Mar. 1, 1994 and entitled "Method and Apparatus for Dynamic Low Voltage Spindle Motor Operation" to Dunfield which is herein incorporated by reference. This application disclosed a method of controlling the spindle motor, specifically a method and apparatus for reconfiguring the spindle motor operation from line-to-line operation to line-to-neutral operation after start-up. In this way, the supply voltage could be maintained at a lower level having only to overcome only a single winding contribution of back EMF.
Often the so called back electromotive force ("EMF") generated by the spindle motor 8 is coupled during power down to the voice coil mechanism to assist in the positioning of the actuator carriage 12. U.S. Pat. No. 4,742,410 to Smith disclosed a method and apparatus for using the spindle motor of a magnetic disc drive system as a generator for supplying the power to a stepper motor to position the actuator carriage away from the magnetic disc surfaces. Similarly, U.S. Pat. No. 4,866,554 to Stupeck et al. disclosed an apparatus for automatically retracting a head of a disc drive upon power interruption which included using the back EMF of the spindle motor coupled with a charged capacitor to supply the power to the voice coil motor to park the head assembly.
Many disc drive units include stationary ramps (not shown) or other parking regions upon which the read/write head ends of the actuator carriage 12 rest when the spindle motor 8 is not running. However, in order to provide the power to drive the head ends up a ramp or radially across the disc, a sufficient amount of torque must be able to be generated, requiring a large current to be delivered from the spindle motor. Heretofore, the higher current necessarily required the development of a higher back EMF or the use of other external power components. As was described above, the development of a higher back EMF would ordinarily require a larger power source voltage V.sub.cc, which is generally undesirable.
What is needed is a mechanism whereby existing spindle motors can be operated to provide a high torque at power-up, not generate an unmanageable back EMF under normal running velocity conditions so as to allow for lower source voltages V.sub.cc, and yet develop sufficient back EMF at power-down to provide high current to a voice coil to drive the heads of an actuator carriage for parking.